The present invention relates to an end mill having high rigidity and suitable for precision-cutting deep grooves into precision parts such as dies.
Certain conventional end mills have been developed to meet the requirements of having high rigidity and being capable of precision cutting, e.g. deep groove cutting.
Those conventional end mills for deep groove cutting have advantageous designs in which relatively small-sized grooves are required for discharging chips because only a small amount of chip material is generated due to the depth of cut being set to an extremely small dimension. Such depth of cut could be as small as 0.004 mm in the direction normal to the slope of a tapered groove in the case of a tapered groove having a taper angle of 5.degree., which is equivalent to 0.1 mm in the direction of the groove depth.
While having such advantage in design, those conventional deep groove cutting end mills, however, must meet the requirement of extremely high rigidity because the cutting edge portion thereof is made more slender than ordinary end mills due to the characteristics of deep groove cutting.
FIGS. 19 through 22 show a typical conventional tapered end mill arranged in consideration of the above characteristics. This end mill is disclosed in Japanese Utility Model Unexamined Publication (Jitsu-Kai) No. SHO-63-161615.
The tapered end mill shown in FIGS. 19 through 22 comprises a right cylindrical column-shaped tool body 10 having tapered cutting edge portion 12 integrally formed at one end of the tool body 10 so that the diameter thereof decreases toward the one end of the tool body 10. On the circumference of the cutting edge portion 12 are a plurality of spiral circumferential cutting edges 20 formed so that a conical rotary locus "R" is formed around the axis of the tool body. The cutting edge portion 12 further has end cutting edges 22 formed at the end thereof. The cutting edge portion 12 has a cross section formed in the shape of a regular polygon, such as the regular hexagon shown in FIG. 21, or the regular triangle shown in FIG. 22, in which a line formed by intersecting adjacent sides 24 forms the circumferential cutting edge 20. A respective overall length of each of the circumferential cutting edges 20 is arranged to be at least six times the rotational diameter at the one end of the circumferential cutting edges 20.
As shown in FIGS. 23 and 24, certain straight end mills having the cutting edge portion formed in a right cylindrical column shape around the axis of the tool body have exactly the same cross section as shown in FIG. 21. Such straight end mills are also used for cutting high hardness material in addition to the deep groove cutting mentioned above.
Those conventional deep groove cutting end mills are able to conduct stable cutting while avoiding chatter during the deep groove cutting because of the possession of higher rigidity than end mills for conventional use; this is due to the large cross sectional area resulting from the lack of chip discharging grooves on the circumference of the tool and also to a large included angle .theta. of the circumferential cutting edge 20.
As described above, the conventional deep groove cutting end mills have the cross section of the cutting edge portion 12 thereof formed in a regular polygonal shape. This means that the rake angle .gamma. of the circumferential cutting edge 20 at a cross section perpendicular to the axis (hereinafter referred to as the "rake angle") is determined by the cross sectional shape of the cutting edge portion 12 resulting in an angle greatly deviated to the negative side, such as -60.degree. for the cutting edge portion 12 having a cross section of a regular hexagon, -45.degree. for a cross section of a square, and -30.degree. for a cross section of a triangle, in which the number of the circumferential cutting edges is minimal, as shown in FIG. 22. Because of this, the conventional deep groove cutting end mills have a disadvantage in that cutting resistance is increased, thereby tending to decrease the cutting sharpness.
It is understood that the greater the number of corners of the cutting edge portion 12, or the greater the number of edges of the circumferential cutting edges 20, the greater the feed of the tool per revolution can be, which is useful for improving cutting efficiency. A larger number of corners can be also effective for improving tool rigidity because of the increase in the tool's cross sectional area. However, the maximum possible number of corners is practically limited to six (hexagonal cross section) because cutting resistance increases due to the rake angle .gamma. being more deviated to the negative side as the number of edges of circumferential cutting edges 20 increases. Therefore, it was not possible to improve cutting efficiency simply by increasing the number of edges.
Furthermore, the conventional deep groove cutting end mills have another disadvantage in that an optimal relief angle .alpha. can not be determined in accordance with cutting requirements because the relief angle .alpha. of the circumferential cutting edge 20 at a cross section perpendicular to the axis (hereinafter referred to as the "relief angle") is determined by the number of corners of the cutting edge portion 12, whereby the relief angle is 30.degree. for a hexagon, 45.degree. for a square, and 60.degree. for a triangle.